Lewis酸对氮杂环卡宾协同催化体系中反应途径的调控

该研究基于氮杂环卡宾（N-heterocyclic carbene,NHC）新颖的协同催化策略,通过Lewis酸共催化剂调控反应具体途径.从α,β-不饱和醛类化合物出发,立足于多反应位点的高烯醇中间体,与氯化镁协作实现高对映选择性的质子转移历程,构建β-手性酯类产物;在相似的反应体系中与氯化钌协作实现高效的空气氧化,构建α,β-不饱和酯类化合物.这两个迥异的反应途径对底物均有较好的官能团容忍性,以高转化率得到目标产物.     

Highly Selective Hydroboration of Alkenes,Ketones and Aldehydes Catalyzed by a Well‐Defined Manganese Complex

Well‐defined manganese complexes based on inexpensive, readily available ligands, 2,2′:6′,2′′‐terpyridine and its derivatives have been prepared and employed for the selective hydroboration of alkenes, ketones and aldehydes. Highly Markovnikov regioselective hydroboration of styrenes as well as excellent chemoselective hydroboration of ketones over alkenes were achieved, for the first time, by an earth‐abundant manganese catalyst.     

N-杂环卡宾的反应及应用

自从1991年Arduengo第一次分离得到稳定的游离N-杂环卡宾以后,N-杂环卡宾金属络合物的研究在近几年来得到了迅速的发展。N-杂环卡宾的反应性能较高,它们与周期表中几乎所有的元素都能发生反应。N-杂环卡宾金属络合物对许多反应有催化作用,它们是一类有潜在应用价值的催化剂。本文对近年来相关的研究成果进行了综述。     

Gold(I) Complexes Stabilized by Nine- and Ten-Membered N-Heterocyclic Carbene Ligands

Nine- and ten-membered N-heterocyclic carbene (NHC) ligands have been developed and for the first time their gold(I) complexes were synthesized. The protonated NHC pro-ligands 2 a – h were prepared by the reaction of readily available N,N′-diarylformamidines with bis-electrophilic building blocks, followed by anion exchange. In situ deprotonation of the tetrafluoroborates 2 a – h with tBuOK in the presence of AuCl(SMe₂) provided fast access to NHC-gold(I) complexes 3 – 10 . These new NHC-gold(I) complexes show very good catalytic activity in a cycloisomerization reaction (0.1 mol % catalyst loading, up to 100 % conversion) and their solid-state structures reveal high steric hindrance around the metal atom (%V_bur up to 53.0) which is caused by their expanded-ring architecture.     

Single-Electron Transfer Reactions Enabled by N-Heterocyclic Carbene Organocatalysis

Over the past decades, N-heterocyclic carbene (NHC) organocatalysis has undergone a flourish of development on the basis of closed-shell reaction paths. By contrast, the emerging area of single-electron transfer (SET) reactions enabled by NHC catalysis still remain underdeveloped, but offer plenty of opportunities to develop new catalytic modes and useful synthetic methods. A number of interesting transformations were triggered by the SET process from the electron-rich Breslow intermediates to various single-electron acceptors. In additions, recent studies revealed that the Breslow radical cations could also be generated by single-electron reduction of the electron-deficient acyl azolium intermediates. These discoveries open a new avenue for NHC organocatalysis to harness radical reactions. The present review will focus on the exciting advancements in the dynamic area of radical NHC organocatalysis.     

过渡金属N-杂环卡宾配合物合成

由于N-杂环卡宾配体（NHCs）的独特性能,N-杂环卡宾过渡金属配合物在均相催化等方面取得了重要应用,但是其合成方法却发展缓慢。本文综述了N-杂环卡宾过渡金属配合物合成方法的最新研究进展,介绍了富电子烯烃裂解反应、游离NHC直接配位反应、配体底物的脱质子原位反应、卡宾加合物热解反应、金属交换转移反应和C2-X（X为甲基、卤原子或氢原子）键氧化加成反应等合成N-杂环卡宾过渡金属配合物的主要方法,此外本课题组还首次发现了金属粉末法,该法可用于规模化合成铁、钴、镍、铜等第一过渡系金属NHC配合物。     

The Mechanism of N-Heterocyclic Carbene Organocatalysis through a Magnifying Glass

The term “N-Heterocyclic carbene organocatalysis” is often invoked in organic synthesis for reactions that are catalyzed by different azolium salts in the presence of bases. Although the mechanism of these reactions is considered today evident, a closer look into the details that have been collected throughout the last century reveals that there are many open questions and even contradictions in the field. Emerging new theoretical and experimental results offer solutions to these problems, because they show that through considering alternative reaction mechanisms a more consistent picture on the catalytic process can be obtained. These novel perspectives will be able to extend the scope of the reactions that we call today N-heterocyclic carbene organocatalysis.     

NHC/Nickel(II)‐Catalyzed [3+2] Cross‐Dimerization of Unactivated Olefins and Methylenecyclopropanes

Cross‐dimerization of a methylenecyclopropane ( 1 ) and an unactivated alkene ( 2 ) with typical hydroalkenylation reactivity was observed for the first time by using a [NHC‐Ni(allyl)]BAr^F catalyst (NHC=N‐heterocyclic carbene). Results show that the C?C cleavage of 1 did not involve a Ni⁰ oxidative addition, which was crucial in former systems. Thus the method reported here emerges as a complementary method for attaining highly chemo‐ and regioselective synthesis of methylenecyclopentanes ( 3 ) with broad scope. An efficient NHC/Ni^II‐catalyzed rearrangement of 1 leads to the convergent synthesis of 3 in the presence of 2 .     

N-Heterocyclic Carbene Analogues of Nucleophilic Phosphinidene Transition Metal Complexes

Chloride abstraction from the complexes [(η⁶-p-cymene){(IDipp)P}MCl] ( 2 a , M=Ru; 2 b , M=Os) and [(η⁵-C₅Me₅){(IDipp)P}IrCl] ( 3 b , IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaBAr^F) in the presence of trimethylphosphine (PMe₃), 1,3,4,5-tetramethylimidazolin-2-ylidene (^MeIMe) or carbon monoxide (CO) afforded the complexes [(η⁶-p-cymene){(IDipp)P}M(PMe₃)]BAr^F] ( 4 a , M=Ru; 4 b , M=Os), [(η⁶-p-cymene){(IDipp)P}Os(^MeIMe)]BAr^F] ( 5 ) and [(η⁵-C₅Me₅){(IDipp)P}IrL][BAr^F] ( 6 , L=PMe₃; 7 , L=^MeIMe; 8 , L=CO). These cationic N-heterocyclic carbene-phosphinidene complexes feature very similar structural and spectroscopic properties as prototypic nucleophilic arylphosphinidene complexes such as low-field ³¹P NMR resonances and short metal-phosphorus double bonds. Density functional theory (DFT) calculations reveal that the metal-phosphorus bond can be described in terms of an interaction between a triplet [(IDipp)P]⁺ cation and a triplet metal complex fragment ligand with highly covalent σ- and π-contributions. Crystals of the C−H activated complex 9 were isolated from solutions containing the PMe₃ complex, and its formation can be rationalized by PMe₃ dissociation and formation of a putative 16-electron intermediate [(η⁵-C₅Me₅)Ir{P(IDipp)}I][BAr^F], which undergoes C−H activation at one of the Dipp isopropyl groups and addition along the iridium-phosphorus bond to afford an unusual η³-benzyl coordination mode.     

N-杂环卡宾Ru配合物的合成及其在Suzuki偶联反应中的催化应用

合成了两个新的氮杂环卡宾金属钌配合物1和2,通过核磁共振氢谱、核磁共振碳谱、红外光谱和元素分析对其结构进行了表征,同时,X射线单晶衍射确证了配合物2的结构为cis(I)顺式构型.化合物1和2均能在温和的反应条件下有效催化卤代芳烃和苯硼酸的Suzuki偶联反应,并表现出较高的催化活性.     

Air-Stable Oxyallyl Patterns and a Switchable N-Heterocyclic Carbene

Oxyallyl derivatives are typically elusive compounds. Even recently reported “stabilized” 1,3-diaminooxyallyl species are still highly reactive and have short lifetimes at room temperature. Herein, we report the synthesis and preliminary study of mesoionic pyrimidine derivatives that feature 1,3-bis(dimethylamino)oxyallyl patterns with an unprecedented level of stabilization. The latter are not only insensitive towards air and moisture, but they are also compatible with the formation of an ancillary stable N-heterocyclic carbene moiety. As the oxyallyl pattern is proton-responsive, it allows the reversible switching of the electronic properties of the carbene, as a ligand.     

Enantiopure Cycloiridiated Complexes Bearing a Pentahelicenic N-Heterocyclic Carbene and Displaying Long-Lived Circularly Polarized Phosphorescence

A fused π-helical N-heterocyclic carbene (NHC) system was prepared and examined through its diastereoisomerically pure cycloiridiated complexes. The latter display light-green phosphorescence with unusually long lifetimes and circular polarization that depends on both the helical NHC P/M stereochemistry and the iridium Δ/Λ stereochemistry. These unprecedented features are attributed to extended π conjugation within the helical carbenic ligand and efficient helicene-NHC–Ir interaction.     

Radical trans‐Hydroboration of Alkynes with N‐Heterocyclic Carbene Boranes

Hydroboration of internal alkynes with N‐heterocyclic carbene boranes (NHC‐boranes) occurs to provide stable NHC (E)‐alkenylboranes upon thermolysis in the presence of di‐tert‐butyl peroxide. The E isomer results from an unusual trans‐hydroboration, and the E/Z selectivity is typically high (90:10 or greater). Evidence suggests that this hydroboration occurs by a radical‐chain reaction involving addition of an NHC‐boryl radical to an alkyne to give a β‐NHC‐borylalkenyl radical. Ensuing hydrogen abstraction from the starting NHC‐borane provides the product and returns the starting NHC‐boryl radical. Experiments suggest that the observed trans‐selectivity results from kinetic control in the hydrogen‐transfer reaction.     

N-Heterocyclic Carbenes Catalyzed Phospho-Aldol Reaction of Aldehydes

An efficient phospho-aldol reaction of aldehydes catalyzed by N-heterocyclic carbenes （NHCs） has been developed. With 10 mol% stable NHC 1,3-bis（2,6-diisopropylphenyl）imidazol-2-ylidene, various aldehydes reacted with dialkylphosphites smoothly to provide a-hydroxy phosphonates in 59%--99% yield. In this process, NHC was assumed to function as a carbon-centered bronsted base.     

Evaluation of Ruthenium(II) N-Heterocyclic Carbene Complexes as Antibacterial Agents and Inhibitors of Bacterial Thioredoxin Reductase

A series of ruthenium(II) complexes with N-heterocyclic carbene (NHC) ligands of the general type (arene)(NHC)Ru(II)X₂ (where X = halide) was prepared, characterized, and evaluated as antibacterial agents in comparison to the respective metal free benzimidazolium cations. The ruthenium(II) NHC complexes generally triggered stronger bacterial growth inhibition than the metal free benzimidazolium cations. The effects were much stronger against Gram-positive bacteria (Bacillus subtilis and Staphylococcus aureus) than against Gram-negative bacteria (Escherichia coli, Acinetobacter baumannii, Pseudomonas aeruginosa), and all complexes were inactive against the fungus Candida albicans. Moderate inhibition of bacterial thioredoxin reductase was confirmed for selected complexes, indicating that inhibition of this enzyme might be a contributing factor to the antibacterial effects.     

铂、铑N-杂环卡宾配合物在硅氢加成反应中的应用*

铂和铑的卡宾金属配合物作为催化剂应用于各种不饱和化合物的硅氢加成反应,表现出非常优良的催化性能和稳定的物理、化学性质,受到了化学工作者的广泛关注。本文对铂、铑N-杂环卡宾金属化合物作为催化剂在催化酮、炔烃、烯烃以及其它不饱和化合物硅氢加成反应中的应用作了介绍,并分析了该类催化剂在有机硅化学领域的应用前景。     

Mixture of Azolium Tetraphenylborate with Isopropylthioxanthone: A New Class of N-Heterocyclic Carbene (NHC) Photogenerator for Polyurethane,Polyester, and ROMP Polymers Synthesis

In the search of smarter routes to control the conditions of N-heterocyclic carbene (NHCs) formation, a two-component air-stable NHC photogenerating system is reported. It relies on the irradiation at 365 nm of a mixture of 2-isopropylthioxanthone (ITX) with 1,3-bis(mesityl)imidazoli(ni)um tetraphenylborate. The photoinduced liberation of NHC is evidenced by reaction with a mesitoyl radical to form an NHC-radical adduct detectable by electron spin resonance spectroscopy. The NHC yield can be determined by ¹H NMR spectroscopy through the formation of a soluble and stable NHC–carbodiimide adduct. To deprotonate the azolium salt and liberate the NHC, a mechanism is proposed in which the role of base is played by ITX radical anion formed in situ by a primary photoinduced electron-transfer reaction between electronically excited ITX (oxidant) and BPh₄⁻ (reductant). An NHC yield as high as 70 % is achieved upon starting with a stoichiometric ratio of ITX and azolium salt. Three different photoNHC-mediated polymerizations are described: synthesis of polyurethane and polyester by organocatalyzed step-growth polymerization and ring-opening copolymerization, respectively, and generation of polynorbornene by ring-opening metathesis polymerization using an NHC-coordinated Ru catalyst formed in situ.     

Ruthenium Olefin Metathesis Catalysts Featuring N-Heterocyclic Carbene Ligands Tagged with Isonicotinic and 4-(Dimethylamino)benzoic Acid Rests: Evaluation of a Modular Synthetic Strategy

A modular and flexible strategy towards the synthesis of N-heterocyclic carbene (NHC) ligands bearing Brønsted base tags has been proposed and then adopted in the preparation of two tagged NHC ligands bearing rests of isonicotinic and 4-(dimethylamino)benzoic acids. Such tagged NHC ligands represent an attractive starting point for the synthesis of olefin metathesis ruthenium catalysts tagged in non-dissociating ligands. The influence of the Brønsted basic tags on the activity of such obtained olefin metathesis catalysts has been studied.     

Sterically Demanding AgI and CuI N-Heterocyclic Carbene Complexes: Synthesis,Structures, Steric Parameters,and Catalytic Activity

The synthesis and full characterization of new air-stable Ag^I and Cu^I complexes bearing structurally bulky expanded-ring N-heterocyclic carbene (erNHC) ligands is presented. The condensation of protonated NHC salts with Ag₂O afforded a collection of Ag^I complexes, and their first use as ligand transfer reagents led to novel isostructural Cu^I or Au^I complexes. In situ deprotonation of the NHC salts in the presence of a copper(I) source, provides a library of new Cu^I complexes. The solid-state structures feature large N-C_NHC-N angles (118–128°) and almost identical angles between the aryl groups on the nitrogen atoms and the plane of the N-C-N unit of the carbene (i.e. torsion angles close to 0°). Among the steric parameters, the percent buried volume (%V_bur) values span easily in the 50–57 % range, and that one of (9-Dipp)CuBr complex (%V_bur=57.5) overcomes to other known erNHC–metal complexes reported to date. Preliminary catalytic experiments in the copper-catalyzed coupling between N-tosylhydrazone and phenylacetylene, afforded 76–93 % product at the 0.5–2.5 mol % catalyst loading, proving the stability of Cu^I erNHC complexes at elevated temperatures (100 °C).     

Zinc‐Catalysed Hydroboration of Terminal and Internal Alkynes

A regioselective hydroboration of alkynes has been developed by using commercially available zinc triflate as a catalyst, in the presence of catalytic amount of NaBHEt₃. The reaction tolerates a wide range of terminal alkynes having several synthetically useful functional groups and proceeds regioselectively to furnish hydroborated products in moderate to excellent yields. This system shows moderate chemoselectivity towards terminal C≡C bond over terminal and internal C=C bond and internal C≡C bond.